Oil light is flicking?

Recently I completed a long journey in my Vauxhall Corsa 1.0L. Towards the end of my journey i noticed the oil warning ligh flicking on and of. When i pulled over i noticed that the oil was very low in my engine. I went to a petrol station and bought a small bottle of oil costing £5.99 (approx 1.5L bottle). I completed my journey, and drove for a couple of days without a problem. Today i noticed the light was flicking on and of again. I pulled up again and checked the oil, but this time there seems to be sufficient oil present. What could the problem be? Is it simply a case of adding more oil in, or is the problem more complex? Thanks in advance for your help and advice.